Non-heat treated wire rod excellent in strength and cold workability and method for manufacturing same

ABSTRACT

Disclosed is a non-heat treated wire rod, comprising, as percentage by weight: C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and the balance being Fe and unavoidable impurities, wherein the non-heat treated wire rod includes ferrite and pearlite microstructures, and wherein the phase fraction of the pearlite satisfies the following relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies the following relational expressions 3 and 4. 
         VP   2   /VP   1 ≤1.4   [Relational expression 1]
 
       50≤(15 VP   1   +VP   2 )/16≤70   [Relational expression 2]
 
         DL   1   /DL   2 ≤1.4   [Relational expression 3]
 
       0.1≤(15 DL   1   +DL   2 )/16≤0.3   [Relational expression 4]

TECHNICAL FIELD

The present disclosure relates to a non-heat treated wire rod excellent in strength and cold workability, and a method for manufacturing the same, and more specifically, to a non-heat treated wire rod excellent in strength and cold workability, which may be suitable to be used as a material for mechanical parts, and a method for manufacturing the same.

BACKGROUND ART

Since cold working methods have a significant effect on reducing heat treatment costs, as well as providing excellent productivity, when compared to hot working methods or mechanical cutting methods, such cold working methods have been widely used in the manufacture of mechanical parts, such as nuts and bolts.

To manufacture mechanical parts using cold working methods as described above, it is essential that the cold workability of steel materials is excellent, and more particularly, it is necessary for deformation resistance to be low and ductility to be excellent when the steel materials are cold worked. This is because when the deformation resistance of steel is high, the lifespan of tools used during the cold working is reduced, and when the ductility of steel is low, the steel is prone to be split, resulting in defective products.

Accordingly, conventional steel materials for cold working may be subjected to a spherodizing annealing heat treatment before cold working. This is because at the time of the spherodizing annealing heat treatment, steel materials are softened to reduce deformation resistance and to increase ductility, thereby improving cold workability. However, since this case incurs additional expenses and degrades manufacturing efficiency, the development of a non-heat treated wire rod that may ensure excellent cold workability without an additional heat treatment is required.

Nevertheless, it is known that when a pearlite fraction of conventional medium carbon steel, containing carbon in an amount of 0.3 wt % or more, exceeds 50%, cold workability is degraded due to reinforcement of a matrix by pearlite microstructures. In particular, when segregation promoting elements, such as Mn, Cr, and the like, are used together to ensure strength, the deviation between a center segregation portion and a non-segregation portion of the medium carbon steel may increase, and such deviation may further increase in non-heat treated steel, ensuring strength by drawing working, thereby having difficulty in achieving cold forging characteristics. In high-strength non-heat treated steel, having a higher level of strength than medium carbon steel, the influence of oxide-based nonmetallic inclusions in a center portion thereof may significantly increase, in addition to microstructure imbalance caused by the segregation of the center portion.

Furthermore, when the segregation of the center portion causes matrix reinforcement, the sensitivity of such nonmetallic inclusions may further increase, thus affecting cold workability. Thus, in the development of high-strength non-heat treated steel having a higher level of strength than medium carbon steel, the deviation between the microstructures caused by the segregation of the center portion, and the influence of the inclusions of the center portion should be examined.

DISCLOSURE Technical Problem

An aspect of the present disclosure may provide a non-heat treated wire rod that may ensure excellent strength and cold forging characteristics without an additional heat treatment, and a method for manufacturing the same.

Technical Solution

According to an aspect of the present disclosure, a non-heat treated wire rod includes: by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities, in which the non-heat treated wire rod includes ferrite and pearlite microstructures, and in which the phase fraction of the pearlite satisfies relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies relational expressions 3 and 4.

VP ₂ /VP ₁≤1.4   [Relational expression 1]

50≤(15VP ₁ +VP ₂)/16≤70   [Relational expression 2]

DL ₁ /DL ₂≤1.4   [Relational expression 3]

0.1≤(15DL ₁ +DL ₂)/16≤0.3,   [Relational expression 4]

where VP₁ and VP₂, respectively, refer to, in a cross-section perpendicular to the longitudinal direction of the wire rod, a pearlite fraction (area %) in the region from the surface of the wire rod to a ⅜ D position in the diameter (D) direction of the wire rod, and a pearlite fraction (area %) in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod, and DL₁ and DL₂, respectively, refer to, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average lamellar spacing (μm) of the pearlite in the region from the surface of the wire rod to the ⅜ D position in the diameter (D) direction of the wire rod, and the average lamellar spacing (μm) of the pearlite in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod.

According to another aspect of the present disclosure, a method for manufacturing a non-heat treated wire rod includes: heating, at a heating temperature of 1,200-1,300° C., a bloom comprising, by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities, and having a carbon equivalent of 0.6 or more and 0.7 or less, maintaining the bloom at the heating temperature for 240 minutes or more, and subjecting the bloom to steel rolling to obtain a billet; reheating the billet, and then subjecting the billet to wire rod rolling to obtain a wire rod; and coiling the wire rod, and then cooling the wire rod at a rate of 0.3-1° C./s.

Advantageous Effects

According to an exemplary embodiment in the present disclosure, a high-strength non-heat treated wire rod that may sufficiently suppress deformation resistance at the time of cold working, even when a spheroidizing annealing heat treatment is removed, may be provided.

BEST MODE FOR INVENTION

The present inventors conducted examinations from all viewpoints, in order to provide wire rods that may ensure excellent cold workability, while having certain levels of strength and hardness, after drawing working. As a result, the inventors found out that high-strength wire rods, for which cold workability was not degraded after drawing working, may be provided by ensuring two phases of ferrite and pearlite with microstructures of a wire rod of medium carbon steel through optimization of alloy compositions of the wire rod and a method for manufacturing the same, and by properly controlling the phase fraction of pearlite, the lamellar spacing of the pearlite, and the like, in each portion of the wire rod, thereby reaching completion of the invention.

Hereinafter, a non-heat treated wire rod excellent in strength and cold workability, according to an aspect of the present disclosure, will be described in detail.

First, alloy elements and composition ranges of the non-heat treated wire rod will be described in detail. All of contents of the respective elements to be mentioned below are based on weight %, unless otherwise stated.

C: 0.3-0.4%

Carbon (C) may serve to improve strength of a wire rod. In an exemplary embodiment in the present disclosure, it may be preferable that a content of C be included in an amount of 0.3% or more, in order to exhibit such an effect. However, when the C content is excessive, deformation resistance to steel may increase rapidly, thus degrading cold workability. Thus, it may be preferable that an upper limit of the C content be 0.4%.

Si: 0.05-0.3%

Silicon (Si) may be an element useful as a deoxidizer. In an exemplary embodiment in the present disclosure, it may be preferable that a content of Si be included in an amount of 0.05% or more, in order to exhibit such an effect. However, when the Si content is excessive, deformation resistance to steel may increase rapidly due to solid solution strengthening, thus degrading cold workability. Thus, it may be preferable that an upper limit of the Si content be 0.3%, more preferably 0.25%.

Mn: 0.8-1.8%

Manganese (Mn) may be an element useful as a deoxidizer or a desulfurizer. In an exemplary embodiment in the present disclosure, it may be preferable that a content of Mn be included in an amount of 0.8% or more, more preferably 1.0% or more, in order to exhibit such an effect. However, when the Mn content is excessive, deformation resistance of steel may increase rapidly due to an excessively high level of strength of the steel itself, thus degrading cold workability. Thus, it may be preferable that an upper limit of the Mn content be 1.8%, more preferably 1.6%.

Cr: 0.5% or less (Including 0%)

Chromium (Cr) may serve to promote ferrite and pearlite transformation at the time of hot rolling. Further, Cr may allow a carbide to precipitate in steel, while not increasing strength of the steel itself to a required level or higher, to reduce an amount of solid solubilized C, contributing to a reduction in dynamic strain aging by the solid solubilized C, but even when Cr is not added, it may not be greatly difficult to secure physical properties. However, when the Cr content is excessive, deformation resistance to steel may increase rapidly due to an excessively high level of strength of the steel itself, thus degrading cold workability. Thus, it may be preferable that the Cr content be 0.5% or less, more preferably 0.4% or less.

P: 0.02% or Less

Phosphorus (P) as unavoidably contained impurities may be an element that may be a primary cause of degrading toughness of steel by segregating to grain boundaries, and reducing delayed fracture resistance. Thus, it may be preferable that a content of P be adjusted to be as low as possible. It may be advantageous that a theoretical content of P is controlled to be 0%. However, P may be inevitably contained in a steel manufacturing process. Thus, it may be important to maintain an upper limit of the P content, and in an exemplary embodiment in the present disclosure, the upper limit of the P content may be maintained to be 0.02%.

S: 0.02% or Less

Sulfur (S) as unavoidably contained impurities may be an element that may be a primary cause of significantly degrading ductility of steel by segregating to grain boundaries, and reducing delayed fracture resistance, and stress relaxation characteristics by forming emulsion in the steel. Thus, it may be preferable that a content of S be adjusted to be as low as possible. It may be advantageous that a theoretical content of S is controlled to be 0%. However, S may be inevitably contained in a steel manufacturing process. Thus, it may be important to maintain an upper limit of the S content, and in an exemplary embodiment in the present disclosure, the upper limit of the S content may be maintained to be 0.02%.

sol.Al: 0.01-0.05%

Soluble aluminum (sol.Al) may be an element useful as a deoxidizer, and may be added in an amount of 0.01% or more, preferably 0.015% or more, and more preferably 0.02% or more. However, when a content of sol.Al exceeds 0.05%, an effect of refining austenite particles due to AlN formation may be great, thereby degrading cold workability. Thus, in an exemplary embodiment in the present disclosure, an upper limit of the sol.Al content may be maintained to be 0.05%.

N: 0.01% or Less

Nitrogen (N) may be unavoidably contained impurities. When a content of N is excessive, deformation resistance to steel may increase rapidly due to an increase in an amount of solid solubilized N, thus degrading cold workability. It may be advantageous that a theoretical content of N is controlled to be 0%. However, N may be inevitably contained in a steel manufacturing process. Thus, it may be important to maintain an upper limit of the N content, and in an exemplary embodiment in the present disclosure, the upper limit of the N content may be preferably maintained to be 0.01%, more preferably 0.008%, and most preferably 0.007%.

O: 0.0001-0.003%

Oxygen (O) may be present within a wire rod in the form of a nonmetallic inclusion, and may be conventionally contained in an amount of 0.0001% or more. However, such a nonmetallic inclusion may be a starting point of a fracture to degrade fatigue strength and cold forging characteristics of steel, and in particular, when strength is secured by drawing working as in non-heat treated steel, fractures may be likely to occur in a center portion of the wire rod, with the nonmetallic inclusion as the starting point. In particular, according to research results obtained by the present inventors, an amount of a nonmetallic inclusion may increase in a wire rod having an O content of more than 0.003% in steel, so that disconnection avoidance may not be sufficient in a workpiece used for strict uses. Thus, in an exemplary embodiment in the present disclosure, an upper limit of the O content may be preferably maintained to be 0.003%, more preferably 0.001%, and most preferably 0.0008%.

At Least One of Nb: 0.005-0.03% and V: 0.05-0.3%

Niobium (Nb) may be an element serving to form a carbonitride to restrict movements of austenite and ferrite within grain boundaries, and may be added in an amount of 0.005% or more. However, the carbonitride may act as a starting point of fractures to degrade impact toughness, in particular, low-temperature impact toughness, and may also be preferably added maintaining a solubility limit thereof. Furthermore, when a content of Nb is excessive, it may exceed the solubility limit, and thus a coarse precipitate may be formed. Thus, it may be preferable that the Nb content be limited to 0.03% or less.

Meanwhile, vanadium (V) may be an element serving to form a carbonitride as Nb to restrict movements of austenite and ferrite within grain boundaries, and may be added in an amount of 0.05% or more. Merely, the carbonitride may act as a starting point of fractures to degrade impact toughness, in particular, low-temperature impact toughness, and may also be preferably added maintaining a solubility limit thereof. Thus, it may be preferable that a content of V be limited to 0.3% or less.

The balance other than the alloy compositions may be iron (Fe). In addition, the non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, may include other impurities that may be included in a conventional industrial steel production process. Since these impurities can be understood by a person skilled in the art, types and contents of the impurities are not particularly limited in an exemplary embodiment in the present disclosure.

Merely, since Ti corresponds to a representative impurity with a content that may be required to be suppressed to the maximum, in order to obtain effects according to an exemplary embodiment in the present disclosure, a brief description thereof is as follows.

Ti: 0.005% or Less

Titanium (T) as a carbonitride formation element may form a carbonitride at a temperature higher than that at which Nb and V may form a carbonitride. Thus, when Ti is included in steel, it may be advantageous to fix C and N, but Nb and/or V may be precipitated using the Ti carbonitride as a core, so that a large amount of coarse carbonitrides may be formed within a matrix, thus degrading cold workability. Thus, it may be important to maintain an upper limit of a content of Ti, and in an exemplary embodiment in the present disclosure, the upper limit of the Ti content may be preferably maintained to be 0.005%, more preferably 0.004%.

According to an example, a carbon equivalent (Ceq) of the non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, may be 0.6 or more and 0.7 or less. Here, the carbon equivalent (Ceq) may be defined by the following formula 1. When the carbon equivalent (Ceq) is less than 0.6 or greater than 0.7, it may be difficult to secure target strength.

Ceq=[C]+[Si]/9+[Mn]/5+[Cr]/12,   [Formula 1]

where [C], [Si], [Mn], and [Cr] each refer to the content (wt %) of a corresponding element.

The non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, may include ferrite and pearlite as microstructures thereof.

The non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, may have a phase fraction of pearlite (volume %) satisfying the following relational formulae 1 and 2.

VP ₂ /VP ₁≤1.4   [Relational expression 1]

50≤(15VP ₁ +VP ₂)/16≤70,   [Relational expression 2]

where VP₁ and VP₂ may, respectively, refer to, in a cross-section perpendicular to the longitudinal direction of the wire rod, a pearlite fraction (area %) in the region from the surface of the wire rod to the ⅜ D position in the diameter (D) direction of the wire rod, and a pearlite fraction (area %) in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod.)

In relational expression 1, as a pearlite phase fraction-related control formula in each portion of the wire rod, in general, when segregation promoting elements, such as Mn and Cr, are actively used in medium carbon steel as in an exemplary embodiment in the present disclosure, the deviation between a center segregation portion and a non-segregation portion of the medium carbon steel may significantly increase, and such deviation may further increase in non-heat treated steel, ensuring strength by drawing working, thereby resulting in a deterioration in cold workability. In an exemplary embodiment in the present disclosure, excellent cold workability may be secured by controlling a value of VP₂/VP₁ to be 1.4 or less.

Meanwhile, since a method of controlling the value of VP₂/VP₁ to be 1.4 or less, as described above, is various, independent claims of the present disclosure do not particularly limit the method. Merely, as an example, the value of VP₂/VP₁ may be controlled to be 1.4 or less by properly controlling a bloom heating temperature and a maintaining time, as described below.

In relational expression 2 as the average pearlite phase fraction-related control formula of the wire rod, when a value of (15VP₁+VP₂)/16 is less than 50 or greater than 70, it may be difficult to simultaneously secure target cold workability and strength.

Further, the non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, may have the average lamellar spacing (μm) of the pearlite satisfying the following relational formulae 3 and 4.

DL ₁ /DL ₂≤1.4   [Relational expression 3]

0.1≤(15DL ₁ +DL ₂)/16≤0.3,   [Relational expression 4]

where DL₁ and DL₂ may, respectively, refer to, in the cross-section perpendicular to the longitudinal direction of the wire rod, the lamellar spacing (μm) of the pearlite in the region from the surface of the wire rod to the ⅜ D position in the diameter (D) direction of the wire rod, and the lamellar spacing (μm) of the pearlite in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod.

In relational expression 3 as a pearlite lamellar spacing-related control formula in each portion of the wire rod, the pearlite lamellar spacing, as well as the pearlite fraction, may have a significant influence on the physical properties of medium carbon steel actively using the pearlite microstructure That is, as the lamellar spacing is finer, strength of the wire rod may increase, and as the difference between the lamellar spacings of the center segregation portion and the non-segregation portion increases, the deviation between the physical properties may be extreme. In an exemplary embodiment in the present disclosure, excellent cold workability may be secured by controlling a value of DL₁/DL₂ to be 1.4 or less.

Meanwhile, since a method of controlling the value of DL₁/DL₂ to be 1.4 or less, as described above, is various, the independent claims of the present disclosure do not particularly limit the method. Merely, as an example, the value of DL₁/DL₂ may be controlled to be 1.4 or less by properly controlling a wire rod rolling temperature and a cooling rate, as described below.

In relational expression 4 as the average lamellar spacing-related control formula of the wire rod, when a value of (15DL₁+DL₂)/16 is less than 0.1 or greater than 0.3, it may be difficult to simultaneously secure target cold workability and strength.

According to an example, a deviation in strength of the pearlite may satisfy relational expression 5.

(VP ₂ /VP ₁)×(√(DL ₁ /DL ₂))≤1.5   [Relational expression 5]

As mentioned above, in general, when Mn and Cr are actively used in non-heat treated medium carbon steel in order to secure strength and cold workability, the deviation between the physical properties across the cross section of the wire rod may be caused by segregation of center portions of Mn and Cr, and may further increase after drawing working, thereby significantly increasing the possibility of an occurrence of internal cracking at the time of forging working for the manufacture of a final product. Relational expression 5 may be the strength deviation-related control formula of the pearlite in each portion of the wire rod, and the present inventors confirmed that molding through cold forging might be possible without the occurrence of internal cracking, irrespective of a large degree of drawing working, even when a value of (VP₂/VP₁)×(√(DL₁/DL₂)) is 1.5 or less, through a number of experiments.

According to an example, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average composition of an oxide-based inclusion in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod may satisfy relational expressions 6 to 8.

30≤[Al₂O₃]≤70   [Relational expression 6]

20≤[SiO₂]≤40   [Relational expression 7]

10≤[CaO]+[MgO]≤20,   [Relational expression 8]

where [Al₂O₃], [SiO₂], [CaO], and [MgO] each refer to the content (wt %) of a corresponding inclusion.

Here, the reason for controlling the composition of a nonmetallic inclusion is to provide a wire rod having further improved drawability and cold workability when the wire rod continues to be drawn by reducing an amount of a hard inclusion (an inviscid inclusion) within the wire rod to the minimum. In particular, the present inventors confirmed that when a content of a specific oxide of an oxide-based inclusion unavoidably mixed in steel increases, the inclusion may be hardened, thereby degrading cold workability.

The reason or the like for determining a content of each oxide forming the oxide-based inclusion will hereinafter be described in detail. The combination of polyvalent oxide compositions may be necessary, in order to reduce the desired number of inviscid inclusions and soften the inviscid inclusions in an exemplary embodiment in the present disclosure. First, the combination of trivalent or more oxides, including at least one of CaO or MgO while necessarily including Al₂O₃ and SiO₂, was found to be optimal.

Al₂O₃: 30-70%

Al₂O₃ may be an element useful to allow an oxide-based inclusion to have a lower melting point and to be softened. It has been known that Al₂O₃ is inevitably present in steel or slag, but when an amount of Al₂O₃ in the slag is properly maintained, the melting point of the inclusion may be lowered, which may allow elongation to be secured to refine the inclusion in a rolling process and may be advantageous in integrity of a final material. In order to effectively exhibit the effect, a content of Al₂O₃ may be adjusted to 30% or more, preferably 35% or more, and more preferably 40% or more. However, when the Al₂O₃ content increases excessively, an alumina-based inclusion, difficult to be refined due to being hard, may be formed, and may also be difficult to be refined in a hot rolling process, thereby being a starting point of fracture or damage. Thus, an upper limit of the Al₂O₃ content may be adjusted to 70%, preferably 65%, and more preferably 60%.

SiO₂: 20-40%

SiO₂ may be inevitably present in steel or slag, together with the above-mentioned Al₂O₃, and may be an important oxide underlying a polyvalent oxide. When a content of SiO₂ is less than 20%, an excellent combination of SiO₂ as the inclusion of the polyvalent oxide and other oxides may not be obtained, and when the SiO₂ content exceeds 40%, it may be highly likely to form a hard inclusion. Thus, it may be preferable to adjust a lower limit of the SiO₂ content to 20%, an upper limit to 40%.

CaO+MgO: 10-20%

MgO and CaO may be elements required to form an inclusion with an optimal composite composition so that a melting point of the inclusion may be lowered. All of MgO and CaO may have a high melting point alone, but may have an effect of lowering the melting point of a polyvalent oxide. In order to exhibit the effect, MgO and CaO may be required to be contained in a total amount of 10% or more. However, when the sum of the contents is excessive, the melting point of the inclusion may rise, or crystals of MgO and CaO may be generated, to render the inclusion difficult to be refined in a hot rolling process, so that the inclusion may be a starting point of fracture or damage. Thus, an upper limit of a CaO+MgO content may be adjusted to a total amount of 20% or less.

According to an example, an average diameter of the oxide-based inclusion may be 8 μm or less (excluding 0 μm), and a maximum diameter of the oxide-based inclusion may be 15 μm (excluding 0 μm).

By refining the nonmetallic inclusion formed of the oxide, as described above, the number of fracture starting points may be reduced. Here, the average diameter and the maximum diameter of the nonmetallic inclusion may refer to the average or maximum equivalent circular diameter of particles detected by observing one cross section in the longitudinal direction of the wire rod, and the maximum diameter of the nonmetallic inclusion was calculated as follows. The nonmetallic inclusion was observed at 400 magnitudes in 800 fields of view by an optical microscope, the maximum diameter of the nonmetallic inclusion in each field of view was marked on gumble probability paper, and an extreme value of about 50,000 mm² was calculated as a maximum diameter.

Meanwhile, since a method of controlling the average composition and diameter of the oxide-based inclusion, as described above, is various, an exemplary embodiment in the present disclosure does not particularly limit the method. Merely, as an example, the average composition and diameter of an oxide-based inclusion formed may be controlled by adjusting the concentrations of dissolved Al and Si and the concentrations of dissolved Mg and Ca in molten steel.

The non-heat treated wire rod, according to an exemplary embodiment in the present disclosure, as described above, may be manufactured by various methods, and a method for manufacturing the same is not particularly limited. Merely, as an exemplary embodiment, the non-heat treated wire rod may be manufactured by the following method.

Hereinafter, a method of manufacturing a non-heat treated wire rod excellent in strength and cold workability, according to another aspect of the present disclosure, will be described in detail.

First, after a bloom, satisfying the above component system, is heated, the bloom may be rolled into billets.

It may be preferable that a heating temperature of the bloom be from 1,200 to 1,300° C., more preferably from 1,200 to 1,250° C. When the heating temperature of the bloom is less than 1,200° C., hot rollability may be degraded, and furthermore, segregation promoting elements of a center portion thereof, such as C, Mn, and Cr, may not be sufficiently diffused, so that the deviation between the microstructures of a segregation portion and a non-segregation portion may increase, thereby causing a deterioration in cold workability. On the other hand, when the heating temperature exceeds 1,300° C., ductility may be degraded due to coarsening of austenite.

According to an example, when the bloom is heated, a maintaining time for which the bloom is maintained at the heating temperature may be 240 minutes or more. When the maintaining time is less than 240 minutes, a homogenization treatment may not be sufficiently performed. Meanwhile, as the maintaining time at the heating temperature increases, it may be advantageous in homogenization, thereby readily reducing an amount of segregation. Thus, an upper limit of the maintaining time is not particularly limited in an exemplary embodiment in the present disclosure.

Subsequently, after the billets are reheated and then rolled into a wire rod, a non-heat treated wire rod may be obtained.

It may be preferable that a reheating temperature for the billets be from 1,050 to 1,250° C., more preferably from 1,100 to 1,200° C. When the reheating temperature for the billets is less than 1,050° C., hot deformation resistance may increase to cause a deterioration in productivity. On the other hand, when the reheating temperature exceeds 1,250° C., ferrite crystal grains may be excessively coarsened. Thus, ductility may be degraded.

According to an example, when the billets are reheated, a maintaining time at which the billets are maintained at the reheating temperature may be from 60 to 240 minutes. When the maintaining time is less than 60 minutes, a homogenization treatment may not be sufficiently performed. Meanwhile, a longer maintaining time at the reheating temperature may be advantageous in homogenization of the segregation promoting elements, but austenite microstructures may be excessively grown, thereby degrading ductility. Thus, an upper limit of the maintaining time may be limited to 240 minutes.

When the wire rod is rolled, a finish rolling temperature may be from 750 to 900° C., preferably from 800 to 880° C. When the finish rolling temperature is less than 750° C., deformation resistance may increase due to an increase in strength caused by refining of the ferrite crystal grains. On the other hand, when the finish rolling temperature exceeds 900° C., the ferrite crystal grains may be excessively coarsened, thereby degrading ductility, and the lamellar spacing of ferrite may be refined, thereby degrading cold workability.

Thereafter, the non-heat treated wire rod may be coiled and then cooled.

According to an example, a coiling temperature for the non-heat treated wire rod may be from 750 to 900° C., more preferably from 800 to 850° C. When the coiling temperature is less than 750° C., martensite generated on the surface layer portion at the time of cooling may not be recovered by a recuperative temperature, and tempered martensite may be generated to form hard, soft steel. Thus, cold workability may be degraded. On the other hand, when the coiling temperature exceeds 900° C., thick scales may be formed on the surface so that a trouble may easily occur when the scales are removed, and in addition, that a cooling time may increase, thereby degrading productivity.

When the non-heat treated wire rod is cooled, a cooling rate may be from 0.3 to 1° C./s, preferably from 0.3 to 0.8° C./s. This is to stably form two phases of ferrite and pearlite. When the cooling rate is less than 0.3° C./s, the lamellar spacing of ferrite microstructures may increase, and thus ductility may be insufficient, and when the cooling rate exceeds 1° C./s, a ferrite fraction may be reduced, and the lamellar spacing of the pearlite may be refined, and thus degrading cold forging characteristics.

MODE FOR INVENTION

Hereinafter, an exemplary embodiment in the present disclosure will be described in more detail with reference to the following Examples. However, the disclosure of such examples is only an example of the implementation of an exemplary embodiment in the present disclosure, and does not limit the present disclosure. This is because the scope of the invention is determined based on the subject matter claimed in the appended claims, and the modifications rationally derived therefrom.

EXAMPLES

A bloom, having an alloy composition as shown in Table 1 below, was heated at 1,250° C. for 5 hours, and then rolled into billets under a finish rolling temperature condition of 1,150° C. Thereafter, the billets were heated at 1,200° C. for 3 hours, and then hot rolled to have a diameter of Φ25 mm, thereby manufacturing a wire rod. At this time, a finish rolling temperature was constantly adjusted to 850° C., and a rolling ratio to 80%. Subsequently, the wire rod was coiled at a temperature of 800° C., and then cooled at a rate of 0.5° C./s.

Subsequently, the pearlite fraction and lamellar spacing of the cooled wire rod, and the composition and size of an inclusion were measured and are shown in Tables 2 and 3 below.

Further, the cold workability of the cooled wire rod was evaluated and is shown in Table 4 below. The cold workability was evaluated based on the presence or absence of cracking by performing a compression test at a true strain of 0.7 on a notch-compressed specimen, and when cracking did not occur, the cold workability was evaluated as “GO,” while when cracking did occur, the cold workability was evaluated as “NG.”

Meanwhile, amounts of drawing working of 10%, 15%, and 20% were applied to respective wire rods, respectively, to manufacture steel wires, and the cold workability of the manufactured steel wires was evaluated and is shown in Table 4 below. A detailed evaluation method thereof is the same as described above.

TABLE 1 Alloy Composition (wt %) Steel Type C Si Mn P S Cr Al Nb V Ti N O Ceq Inventive 0.30 0.23 1.52 0.011 0.0042 0.00 0.03 0.025 0.0042 0.0007 0.630 Stee1 1 Inventive 0.33 0.21 1.48 0.011 0.0044 0.25 0.03 0.11 0.0045 0.0008 0.670 Stee1 2 Inventive 0.35 0.17 1.33 0.010 0.0055 0.13 0.02 0.010 0.12 0.0044 0.0010 0.646 Stee1 3 Inventive 0.37 0.16 1.26 0.012 0.0043 0.11 0.04 0.09 0.003 0.0052 0.0005 0.649 Stee1 4 Inventive 0.39 0.15 1.02 0.010 0.0052 0.00 0.02 0.008 0.11 0.002 0.0044 0.0011 0.611 Stee1 5 Comparative 0.32 0.26 1.69 0.010 0.0058 0.00 0.03 0.023 0.0058 0.0027 0.687 Stee1 1 Comparative 0.34 0.24 1.51 0.010 0.0055 0.34 0.03 0.17 0.0055 0.0025 0.697 Stee1 2 Comparative 0.38 0.18 1.48 0.012 0.0062 0.22 0.02 0.018 0.14 0.0053 0.0019 0.714 Stee1 3 Comparative 0.42 0.16 1.45 0.010 0.0047 0.16 0.03 0.08 0.018 0.0045 0.0011 0.741 Stee1 4 Comparative 0.45 0.17 1.37 0.012 0.0053 0.00 0.02 0.013 0.11 0.015 0.0050 0.0020 0.743 Stee1 5 Here, Ceq = [C] + [Si]/9 + [Mn]/5 + [Cr]/12 [C], [Si], [Mn], and [Cr] each refer to the content (wt %) of a corresponding element.

TABLE 2 Micro- structures Steel Type Type □ □ □ □ □ Note Inventive F + P 1.06 58.3 1.33 0.22 1.22 Inventive Stee1 1 Example 1 Inventive F + P 1.14 60.6 1.28 0.17 1.28 Inventive Stee1 2 Example 2 Inventive F + P 1.20 62.7 1.22 0.19 1.32 Inventive Stee1 3 Example 3 Inventive F + P 1.27 64.1 1.16 0.15 1.36 Inventive Stee1 4 Example 4 Inventive F + P 1.35 66.9 1.05 0.12 1.38 Inventive Stee1 5 Example 5 Comparative F + P 1.17 59.8 1.45 0.23 1.40 Compar- Stee1 1 ative Example 1 Comparative F + P 1.25 61.6 1.41 0.18 1.48 Compar- Stee1 2 ative Example 2 Comparative F + P 1.34 65.3 1.33 0.14 1.54 Compar- Stee1 3 ative Example 3 Comparative F + P 1.46 70.2 1.27 0.12 1.64 Compar- Stee1 4 ative Example 4 Comparative F + P 1.55 72.5 1.19 0.09 1.69 Compar- Stee1 5 ative Example 5 Here, of microstructure types, F refers to ferrite, and P refers to pearlite. Further, {circle around (1)} refers to VP₂/VP₁, {circle around (2)} refers to (15VP₁ + VP₂)/16, {circle around (3)} refers to DL₁/DL₂, {circle around (4)} refers to (15DL₁ + DL₂)/16, and {circle around (5)} refers to (VP₂/VP₁) × (√(DL₁/DL₂)).

TABLE 3 Average Maximum Inclusion Inclusion Inclusion Composition (wt %) Diameter Diameter Steel Type Al₂O₃ SiO₂ CaO MgO Sum (μm) (μm) Note Inventive 64 22 7 6 99 7.1 9.1 Inventive Stee1 1 Example 1 Inventive 55 25 8 5 93 7.5 7.3 Inventive Stee1 2 Example 2 Inventive 40 28 5 7 80 5.8 10.5 Inventive Stee1 3 Example 3 Inventive 36 21 8 8 73 6.5 11.3 Inventive Stee1 4 Example 4 Inventive 32 26 10 4 72 4.6 9.8 Inventive Stee1 5 Example 5 Comparative 82 11 2 3 98 6.2 16.7 Comparative Stee1 1 Example 1 Comparative 63 17 1 5 86 7.6 15.6 Comparative Stee1 2 Example 2 Comparative 52 23 5 2 82 8.8 11.5 Comparative Stee1 3 Example 3 Comparative 37 30 7 3 77 9.4 10.4 Comparative Stee1 4 Example 4 Comparative 22 35 10 5 72 11.3 12.2 Comparative Stee1 5 Example 5

TABLE 4 Cold Workability Wire Steel Wire Steel Wire Steel Wire Steel Type Rod (10%) (15%) (20%) Note Inventive GO GO GO GO Inventive Steel 1 Example 1 Inventive GO GO GO GO Inventive Steel 2 Example 2 Inventive GO GO GO GO Inventive Steel 3 Example 3 Inventive GO GO GO GO Inventive Steel 4 Example 4 Inventive GO GO GO GO Inventive Steel 5 Example 5 Comparative GO GO NG NG Comparative Steel 1 Example 1 Comparative GO GO NG NG Comparative Steel 2 Example 2 Comparative GO GO GO NG Comparative Steel 3 Example 3 Comparative GO GO GO NG Comparative Steel 4 Example 4 Comparative GO GO GO NG Comparative Steel 5 Example 5

As can be seen from Table 4, in the case of Inventive Examples 1 to 8 satisfying the alloy compositions and manufacturing conditions proposed in an exemplary embodiment in the present disclosure, all of the conditions of relational expressions 1 to 5 were satisfied. In addition, the composition, average diameter, and maximum diameter of a nonmetallic inclusion were controlled to the conditions proposed in an exemplary embodiment in the present disclosure, so that cracking did not occur therein after drawing working, thereby securing excellent strength and cold workability. On the other hand, in the case of Comparative Examples 1 to 5, at least one of the conditions, proposed in an exemplary embodiment in the present disclosure, was not satisfied, so that cracking occurred therein after drawing working, thereby degrading cold workability, as compared to the Inventive Examples. 

1. A non-heat treated wire rod comprising: by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities, wherein the non-heat treated wire rod includes ferrite and pearlite microstructures, and the phase fraction of the pearlite satisfies relational expressions 1 and 2, and the average lamellar spacing of the pearlite satisfies relational expressions 3 and
 4. VP ₂ /VP ₁≤1.4   [Relational expression 1] 50≤(15VP ₁ +VP ₂)/16≤70   [Relational expression 2] DL ₁ /DL ₂≤1.4   [Relational expression 3] 0.1≤(15DL ₁ +DL ₂)/16≤0.3,   [Relational expression 4] where VP₁ and VP₂, respectively, refer to, in a cross-section perpendicular to the longitudinal direction of the wire rod, a pearlite fraction (area %) in the region from the surface of the wire rod to a ⅜ D position in the diameter (D) direction of the wire rod, and a pearlite fraction (area %) in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod, and DL₁ and DL₂, respectively, refer to, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average lamellar spacing (μm) of the pearlite in the region from the surface of the wire rod to the ⅜ D position in the diameter (D) direction of the wire rod, and the average lamellar spacing (μm) of the pearlite in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod.
 2. The non-heat treated wire rod of claim 1, wherein a deviation in strength of the pearlite satisfies relational expression
 5. (VP ₂ /VP ₁)×(√(DL ₁ /DL ₂))≤1.5   [Relational expression 5]
 3. The non-heat treated wire rod of claim 1, wherein the unavoidable impurities include Ti, and an amount of Ti is limited to, by wt %, 0.005% or less.
 4. The non-heat treated wire rod of claim 1, wherein a carbon equivalent is 0.6 or more and 0.7 or less.
 5. The non-heat treated wire rod of claim 1, wherein, in the cross-section perpendicular to the longitudinal direction of the wire rod, the average composition of an oxide-based inclusion in the region from the ⅜ D position in the diameter (D) direction of the wire rod to the center of the wire rod satisfies relational expressions 6 to
 8. 30≤[Al₂O₃]≤70   [Relational expression 6] 20≤[SiO₂]≤40   [Relational expression 7] 10≤[CaO]+[MgO]≤20,   [Relational expression 8] where Al₂O₃, SiO₂, CaO, and MgO each refer to the content (wt %) of a corresponding inclusion.
 6. The non-heat treated wire rod of claim 5, wherein the average diameter of the oxide-based inclusion is 8 μm or less.
 7. The non-heat treated wire rod of claim 5, wherein the maximum diameter of the oxide-based inclusion is 15 μm or less.
 8. A method for manufacturing a non-heat treated wire rod comprising: heating, at a heating temperature for 1,200-1,300° C., a bloom comprising, by wt %, C: 0.3-0.4%; Si: 0.05-0.3%; Mn: 0.8-1.8%; Cr: 0.5% or less; P: 0.02% or less; S: 0.02% or less; sol.Al: 0.01-0.05%; N: 0.01% or less; O: 0.0001-0.003%; at least one of Nb: 0.005-0.03% and V: 0.05-0.3%; and a balance of Fe and unavoidable impurities, and having a carbon equivalent of 0.6 or more and 0.7 or less, maintaining the bloom at the heating temperature for 240 minutes or more, and subjecting the bloom to steel rolling to obtain billets; reheating the billets, and then subjecting the billet to wire rod rolling to obtain a wire rod; and coiling the wire rod, and then cooling the wire rod at a rate of 0.3-1° C./s.
 9. The method of claim 8, wherein the unavoidable impurities include Ti, and an amount of Ti is limited to, by wt %, 0.005% or less.
 10. The method of claim 8, wherein a reheating temperature for the billets ranges from 1,050-1,200° C.
 11. The method of claim 8, wherein a coiling temperature for the wire rod ranges from 750-900° C. 